Production of solutions of alkali metal salts of sulphurous acid



Aug. s, 1935.

G. HAGLUND PRODUCTION OF SOLUTIONS .OF ALKALI METAL SALI'SA OFSULPHUROUS ACID Filed Jan. 12, 1933 Irl @m l NS Patented Aug.

meuneries Grstaf Ha'glundl, Sockh 01m, Sweden, assigner `'toPatentaktiebolaget Grndal -Ramem Stock# Application tammy 12, loris,sei-,iai No. 651,245

l In Germany Aira 29,1932;

io claims. (ci,` cs2-liti rllflis invention relates tol the productionoi solutions of alkali rnctal Salts of s'lllllhurousi and has for anobject the provision of` an ini# proved process for producting` suchsolutions.

5- More particularlythe invention contemplates the provision solutionscontaining.

of an improved process'l for' producing alkali metal sulphites and/oralkali metal bisulphitessubstantially fre'e troni calcium salts. The

invention further contemev organic matters of the liquors are burnt;lThe residue may then be freed of its contenty ofsulphides andthiosulphates in a known manner, for example through. roasting wherebythe compounds in question are converted to the corresponding sulphitesor sulphates whereupon the roasted'` material isdissolved; in water,orby placing the residue in solution in water and precipitating the4sulphides and/ or thiosulphates or converting themY to harmlesscompounds such as sul--l phites or sulphates.

Solutions thus produced,` free from sulphides and thiosulphates,containchiefly alkali metal carbonatos, alkaliy metal sulphites and'alkali metal sulphates.

According to the processV of the invention, the alkaliV meta-l salts areconverted to alkali metal bisulphites by iirst `treating-` thef solutionwithsulphu-r dioxide gas andA one or morev calcium compounds capablecalcium carbonate, hydrate oflime in a of reacting with sulphurous acidtoform calcium bisulphite,A for instance,

burnt lime (quick lime)` o1' very finely divided state.

yThe resulting reactions, in case vthe alkali 1 metal salts are sodiumsalts and the added cal'-,

cillin salt is carbonate of calcium are indicated by the followingequations: Y

(l) NazCOzl+H20+2SO2=2NWHSG3+CO2 (e) canetsomcrnezsoizNansoiJfcesoi'rile sulphur'eus acid reacts directly with the alkali metal carbonkcompound .to form calcium bisulphite;

ate' and the` addefd` calcium alljaliy metal bislil'pnite ,and'rhecalcium: blsulpliitef' `iii turn reacts with the alkali metalsulphate forming alkali metal bisulpl'ite and gypsum, the latter beingmainly obtainedas* aprecipitate.

After this treatmerity t'liesolution will contain metal bisulfphiteaeeleitiii bisel'iihita a :i Varying' quantity of free S02, a' quantityof Sol-ved Vvvlii'ch stands relation to the' degree' of solubility ofgypsum in the solution, and` further; suspension or as a precipitate,the i-elniaiiiin'gl gypsum formed, and suchl other insole abre matterses may be present in the solution,

In the i'tt step* of the process, to this s-'o1u.-'' tion from which thesuspended or precipitated riia'tters' have rst been removed, for in=stance by filtering, further quantities of one or 15'` more" finelydivided calcium compounds', s'u'r'zhl als li-l'nef stone', othercal'cillm carbonatos, calcium oxide or calcium hydroxide are addedwithout any further introduction of sulpliur dioxide; and preferablywith powerful vigorous stirring.

The entire free SO2 content of the solution nowcoinliineswith the addedcalcium compounds tor ifoiniA calcil'ln bis'ulphite, and the solutionwill consequently only contain bisulpliites, but nofree Sz. Th-contcntofdissolved'calciul bisulphi'te 2oL willl thus: be increased, i-rthe'solution was not already' saturated' witli calcium tlsuipnite.

The addition of the inely divided fcaloi'llrii compound* isv continued,and, as no free' SO2 is niesentii'ri tlfieseliitieii,theiealeiumcompound 3o* reacts sulphur diide of the bisulphte saltspresent in the solution, whereby monosuh plut-Leer` arel forrr'iet.v Theformationn of monosalpifiitee lie-glas with the ealei'iim Fbieiilp'niteanu tri-eri pioeee'els'to' the alkali metal bi-sulpli'rtes and sti is"equivalent 'tothe added quantities of calcium compound, sotll'at,through al'ljusting`V theV addi-'- tion: of the calcium compound,solutions *can* be obtained containing all the alkali inl` the forni' ofalkalinietali creciente, eillinu'-i'A mete-r mtiiesulpiilte ici oialkali metal biSul/phiteand alkali metal monosulel'iiteiiialdesifedfrelatieri i l is the calcium menesul'p "te is nearly inseln? ble, itprecipitates-frein the solution immediately u eviterhaving" been formed.ni the seme degree 45* as the conversion to mciioeulpnite takesY plate;the solution is freeflireiiiitne enumsalts disselves therein1 ThisVAapplies-L net only teV the calciumjbisulphite, but' also to theirgypsum pres; y ent inf tneisellitieii. frites, if all the calcium bi- 5osulphitej is oil'ivertedv to` calcium4 mbriosulphite and precipitate@from trie solution, and the solution eoiijjequently only contains alkaliinet-'ar tisulpiiite anni'l the alkali metal bi'suipliite will convertedby' the Lalci1ll'm'- compound; that 55 may still be added to alkalimetal monosulphite, whereby ions of monosulphite appear in the solution.As soon as this takes place and due to the fact that calciummonosulphite is much less soluble than gypsum, the ions react with thegypsum forming calcium monosulphite, whereby at the same time alkalimetal sulphate is formed.

After the treatment is nished, the solution will contain a quantity ofalkali metal sulphate equivalent to the precipitated quantity of gypsum.

By means of the above-related treatment with calcium compounds thesolution can be obtained wholly or practically free from calcium salts.

It, after the whole quantity of gypsum has been removed, still morealkali metal bisulphite is to be converted to alkali metal monosulphite,the addition of the calcium compound is continued until the desireddegree of conversion has been obtained. The addition of the calciumcompound may be continued until al1 the bisulphite has been convertedinto monosulphite.

The solution, which may contain alkali metal bisulphites, alkali metalmonosulphites or a mixture of both, and which also may contain smallquantities of gypsum or alkali lmetal sulphate is then,inasuitablemanner, for instance by ltering, separated from the precipitateobtained, consisting of insoluble n lime compounds'. This precipitatemay, to a small degree, consist of gypsum or calciuml compounds whichhave not taken part in the reactions, but chiey consists of calciummonosulphite. Through the above related treatment, a certain quantity ofSO2 combined to the calcium monosulphite is removed from the solutionl:In order that this SO2 should not be wasted, but returned to theprocess, the precipitate taken oi with its contents oi SO2 can beemployed in the preparation of a new quantity of solution, i. e. canserve as a substitute for a part of the calcium compounds required forthis purpose.

The calcium monosulphite of the precipitate combined with the SO2 toform calcium bisulphite in a more rapid way than is, for instance, thecase with calcium carbonate, and then reacts with the alkali metalsulphates in the'above described manner. In this way, consequently, theSO2 combined with the calcium monosulphite is returned to the processWithout any losses.

The accompanying ilowsheet diagrammatically illustrates a preferredmethod of practicing the invention.

Waste liquor such as results from cellulose digestion processes issuitably concentrated, as by evaporation, to remove the bulk of theWater, and is introduced into an incinerator I, where it is heated toremove the balance of the water and to eiect combustion of the organicmatter. -The product from the incinerator is fed into a roasting furnace2, where it is roasted under conditions such that sulphides andthiosulphates contained therein are converted to sulphites and sulphatesThe roasted product, containing substantially no free sulphides orthiosulphates, is transferred to a leaching tank 3, where the alkalimetal salts are dissolved in water and the insoluble matter is separatedout.

VThe aqueous solution thus obtained is introduced into a reaction vessel5. Limestone and/or calcium monosulphite in a nely divided condition isadded, and sulphur dioxide is passed into the solution. The limestoneand/ or calcium monosulphite reacts with alkali metal sulphates landsulphites in the presence of sulphur dioxide to form a solution ofalkali-metal bisulphites and a precipitate of calcium sulphate. Carbondioxide that is evolved during the course of the reaction is allowed toescape.

When substantially all of the alkali metal salts have been converted tobisulphite, the solution is conducted to a settling tank 6 Where theprecipitated calcium sulphate settles out and is removed. The clariedsolution then passes to a precipitating tank l and limestone in a nelydivided state is added. The limestone serves to precipitate as calciummonosulphite the calcium compounds remaining in solution and to free thesolution from sulphur dioxide, the alkali metal bisulphite being at thesame time converted to alkali metal sulphites. The proportion ofbisulphites converted to sulphites may be controlled by regulating thequantity of limestone added. Carbon dioxide is evolved during the courseof the reaction and escapes through a suitable vent.

Solution from the precipitating tank l', containing calcium monosulphitein suspension, passes to va settling tank 8, Where the calciummonosulphite settles and is separated from the residual solution. Theseparated calcium monosulphite may be stored in tank 4, from which itmay conveniently be added to solution contained in the reaction vessel5. Claried alkali metal sulphite solution from the settling tank 8 isconveyed to suitable storage tanks (not shown) from which it may bedistributed to various points of use.

I claim:

1. The process for preparing a solution of alkali metal salts ofsulphurous acid from waste liquor resulting from a cellulose digestionprocess employing cooking liquor containing alkali metal sulphurcompounds, which comprises treating the Waste liquor to eliminateorganic compounds and to form a solution of alkali metal compoundscontaining an alkali metal sulphate and substantially free fromsulphides and thiosulphates, subjecting the solution to the action ofsulphur dioxide and one or more calcium compounds capable of reactingwith sulphurous acid to form calcium bisulphite until the alkali metalsalts contained in the solution have been converted to bisulphites, andsubjecting the bisulphite solution to the actionl of one or more calciumcompounds capable of reacting with bisulphite to form calciummonosulphite until calcium salts in the solution are converted to themonosulphite form, the treatment of the bisulphite solution with thecalcium compounds being regulated to produce a solution substantiallyfree of calcium salts.

2. Process according to claim l in which treatment of the bisulphitesolution with one or more calcium compounds is so regulated thatconversion of any substantial amount of alkali metal bisulphite to themonosulphite form is avoided.

3. Process according to claim 1 in which treatment of the bisulphitesolution with one or more calcium compounds is so regulated that aportion of alkali metal bisulphite is converted to the monosulphiteform.

4. Process according to claim 1 in which treatment of the bisulphitesolution with one or more calcium compounds is so regulated that thealkali metal bisulphite is substantially all converted tothemonosulphite form.

5. Process according to claim 1 in which the calcium monosulphiteproduced is utilized in conjunction with the sulphur dioxide forproducing the bisulphite solution.

6. The process for preparing a solution of alkali metal salts ofsulphurous acid from Waste liquor resulting from a cellulose digestionprocess employing cooking liquor containing alkali metal sulphurcompounds, which comprises treating the waste liquor to eliminateorganic compounds and to form a solution of alkali metal compoundscontaining an alkali metal sulphate and substantially free fromsulphides and thiosulphates, subjecting the solution to the action ofsulphur dioxide and calcium carbonate until the alkali metal saltscontained in the solution have been converted to bisulphites, andsubjecting the bisulphitesolution to the action of calcium carbonateuntil calcium salts in the solution are converted to the monosulphiteform.

7. Process according to claim 6 in which treatment of the bisulphitesolution with calcium carbonate is so regulated that conversion of anysubstantial amount of alkali metal bisulphite to the monosulphite formis avoided.

8. Process according to claim 6 in which treatment of the bisulphitesolution with calcium carbonate is so regulated that a portion of alkalimetal bisulphite is converted to the monosulphite form.

9. Process according to claim 6 in which treatment of the bisulphitesolution with calcium carbonate is so regulated that the alkali metalbisulphite is substantially all converted to the monosulphite form.

10. The process for preparing a solution containing one or more alkalimetal salts of sulphurous acid which comprises subjecting a solutioncontaining an alkali metal sulphate to the action of sulphur dioxide andone or more calcium compounds capable of reacting with sulphurous .acidto form calcium bisulphite until the alkali metal salts contained in thesolution have been converted to bisulphites, and subjecting thebisulphite solution to the action of one or more calcium compoundscapable of reacting with bisulphite to form calcium monosulphite untilcalcium salts in the solution are converted to the monosulphite form,the treatment of the bisulphite solution with the calcium compoundsbeing regulated to produce a solution substantially free of calciumsalts.

GUSTAF HAGLUND.

